1. Field of the Invention
The present invention relates to an improvement in a discharge lamp lighting device, or more in particular to an improvement in a discharge lamp lighting device of a novel type designed to enable a discharge lamp having a lamp voltage approximate to a source voltage to be lighted with a single choke.
2. Description of the Prior Art
In lighting a discharge lamp by use of a prior art lighting device with a single choke, a source voltage sufficiently high as compared with the lamp voltage is required. In recent years, however, a novel type of discharge lamp lighting device (such as disclosed in U.S. Pat. No. 3,997,814) has been suggested in which the discharge lamp can be lighted without intermediary of any step-up transformer by a source voltage as low as the lamp voltage. First, this type of lighting system will be briefly described with reference to FIG. 1. The fundamental circuit of this prior art lighting system comprises an AC power supply 1, a ballast or stabilizer 2 including an inductance and a discharge lamp 3 connected in series to each other, and a switch circuit 4 inserted between terminals A and B in parallel to the discharge lamp 3. The switch circuit 4 is closed (switched on) for a properly predetermined period thereby to store electromagnetic energy in the inductance of the ballast 2 every half cycle of the source voltage, after which the switch circuit 4 is opened (switched off) and the stored electromagnetic energy is superimposed on the source voltage. The resulting voltage is applied to the discharge lamp, thus lighting the same.
According to this lighting system, even a discharge lamp such as a 40 W fluorescent lamp of 105 V lamp voltage which cannot be lighted by an ordinary low-voltage AC power supply (such as a commercial power supply of AC 100 V) can be lighted as rated directly by the commercial power supply of AC 100 V. Further, the lighting device as a whole can be reduced both in size and in weight.
The switch circuit 4 heretofore suggested, however, is what is called the "source voltage in-phase type" in which as shown in FIG. 2 the operating signal for the switch circuit 4 is derived only from the source voltage. Specifically, it is seen from FIG. 2 that the signal S for actuating the switch circuit 4 is generated in phase with the source voltage, as shown in FIG. 3 by a switching signal generator circuit 4' inserted between the terminals a and b of the power supply 1.
As seen from FIG. 3, the phase in which the switch circuit 4 is closed generally corresponds to a time point when the source voltage V.sub.1 is zero. To the extent that an inductance is used as the ballast 2, the discharge lamp 3 is still discharging and therefore the current flowing in the inductance is not yet zero at the phase where the source voltage becomes zero. By shorting the terminals of the discharge lamp 3 by closing the switch circuit 4 under this condition, considerable part of the energy stored in the inductance fails to be effectively applied to the discharge lamp and is wasted through the switch circuit 4, resulting in an insufficient circuit efficiency. Also, in view of the fact that the operation timing of the switch circuit 4 depends on the source voltage V.sub.1, a change in source voltage, ambient temperature or power frequency causes a change in the operation timing of the switch circuit 4 with respect to the actual operating condition of the discharge lamp, thereby changing the lamp output. Furthermore, a considerable power loss occurs in the switch circuit.